Method for alloying highly reactive alloying constituents



United States Patent f 3,492,114 METHOD FOR ALLOYING HIGHLY REACTIVE ALLOYING CONSTITUENTS Hans Schneider, Winterthur, Switzerland, assignor to Sulzer Brothers, Ltd., Winterthur, Switzerland, a corporation of Switzerland No Drawing. Filed Oct. 17, 1967, Ser. No. 675,762 Claims priority, application Switzerland, Oct. 19, 1966,

15,127/ 66 Int. Cl. C21c 7/00 US. Cl. 7553 14 Claims ABSTRACT OF THE DISCLOSURE The metal constituents which are to be alloyed in the alloy or steel melt are added in the form of an oxide to the lining of the treatment vessel containing the melt so that upon addition of lithium or calcium to the melt the lithium or calcium replaces the metal constituents of the oxide to free the melt constituents for alloying with the melt. A second metal constituent can also be added by being directly immersed in the melt in the same manner as the lithium or calcium.

This invention relates to a method for alloying highly reactive alloying constituents to melts of an alloy or of a steel.

Highly reactive alloying constituents such, for example, as titanium, aluminum, zirconium, boron, uranium, niobium and tantalum, readily form oxides, sulphides, 0r nitrides with the impurities of an alloy or steel melt, particularly with the oxygen, sulphur, or nitrogen dissolved in the melts and cannot therefore be easily alloyed thereto.

Previously, due to such reactions, these constituents were necessarily alloyed under vacuum. However, such a procedure has not only required extensive apparatus but also has been awkward and time consuming.

Accordingly, it is an object of the invention to dispense with the need for operating under vacuum when alloying highly reactive alloying constituents into metal melts.

It is another object of the invention to alloy a metal into a metal melt by reduction of an oxide of the alloy metal in the lining of a vessel containing the melt.

Briefly, the method of the invention effects the addition ofa highly reactive alloying metal into a metal melt of an alloy or steel through the use of a vessel lining including one or more of the oxides of the alloying metal and the addition of a reactive metal reducing agent such as, lithium and/or calcium metal, to the melt. Since the lithium and/ or calcium is more highly reactive with oxygen than the alloying metal, the added lithium and/or calcium reacts with the alloying metal oxide to reduce the alloying metal while forming lithium oxide or calcium oxide. Another alloying metal can also be added to the melt along with or after addition of the reactive metal reducing agent in order to further alloy the melt. The quantity of the alloying metal migrating from the vessel lining into the alloy or steel melt is determined by suitable metering of the amount of lithium and/or calcium introduced into the melt.

The oxide or oxides or the alloying metal may be present in the lining of the treatment vessel in proportions of a few percent, for example, up to 100%. Where the oxide or oxides of the alloying metal represents only part of the lining, the parent lining of the treatment vessel is advantageously made of crystalline lime or thorium oxide because these are not attacked by lithium.

Crystalline lime refers to calcium oxide crystallized 3,492,114 Patented Jan. 27, 1970 from the melt (see also, for example, Kristall-Kalk- Kolloquium of the 7th December 1965 by Dynamit Nobel A.G., Feldmiihle-Liilsdorf/Germany), said material having an improved hydration stability and being therefore suitable as a crucible lining material.

Since the vapor pressure of lithium and/or calcium is relatively high at the temperature of the melts, for example, the vapor pressure for lithium at 1650" C. is 6 atm. gauge for lithium, the introduction of lithium or calcium into the melt has been difficult. However, various methods can be employed for this purpose. In a first method, the lithium or calcium is introduced into the alloy or steel melt in an autoclave in which a protective atmosphere is maintained whose pressure exceeds the vapor pressure of the lithium or calcium at the temperature of the melt. The material is added either by at least one immersion of bodies containing metallic lithium and/or calcium, or by blowing lithium and/or calcium powder into the melt by means of an inert entrainment gas, for example, a rare gas or carbon monoxide.

It is also possible for vaporized lithium and/or calcium to be blown into the melt. To this end, lithium or calcium or intermetallic compounds thereof containing lithium or calcium are heated under a protective atmosphere and are vaporized in an autoclave vessel, for example, by means of an induction coil. In addition, the vessel is provided on the underside with nozzle openings which are initially closed. The vapor pressure which increases with an increasing temperature causes evaporated lithium and/or calcium to be discharged in powerful jets from the nozzle after these are opened. These jets, thus, penetrate into the melt disposed in the lower portion of the vessel. Owing to the high velocity of the vapor jets, it is possible in some circumstances to operate without autoclaves in a protective atmosphere, that is, it is not necessary to maintain a pressure over the melt which is higher than the vapor pressure of the lithium or calcium at the temperature of the melt.

A further method for the production of the reactive metal or metals at normal pressure consists in introducing the lithium and/or calcium into the alloy or steel melt by introducing at least one porous carrier body containing lithium and/or calcium. This body can be previously impregnated with lithium and/or calcium melted under vacuum and/or a protective atmosphere at a relatively low temperature, or charged with lithium and/or calcium by combining a compressing and sintering operation.

A porous oxidic carrier body can be produced from calcium oxide or from the oxide of the alloying metal to be alloyed, by calcining near the melting point of the affected oxide. A metallic carrier body of high specific gravity, for example, nickel, cobalt, molybdenum or tungsten can also be used. However, when using metallic carrier bodies, it is important to ensure that the parent substance is compatible in terms of alloying technique with the melt to be treated, that is, that dissolving of the carrier material in the melt is permissible. In some circumstances, it is therefore necessary to employ carrier bodies of different metals, at least part of the carrier bodies being constructed of material which cannot be melted in the melt. On the other hand, the carrier material may be deliberately made of a substance which must be present as a constituent of the final alloy. The introduction of such a carrier body simultaneously adds the constituent concerned.

As already known, lithium and/or calcium may be added to the melt in all cases under a protective atmosphere. Furthermore, the alloy or steel melt can be rabbled during treatment by means of a low frequency multiphase alternating current generated by rabbling coils in order to ensure improved penetration of the reactive metal and good mixing of the melt.

Further objects and advantages of the invention will become more apparent from the following detailed description of two exemplified embodiments of the invention.

EXAMPLE I A nickel alloy 713C containing as precipitation hardener 6% of aluminum and 0.6% of titanium, both materials being suitable for alloying in accordance with the method described in the invention is to be produced.

The alloying constituents, nickel, chromium, molybdenum and niobium/tantalum are melted in accordance with their weight proportion in the alloy in a medium frequency induction furnace lined with MgAl spinel, or in some other known manner, if necessary by using some other suitable lining. Thereafter, the molten material is teemed into a suitably lined treatment vessel which is provided with rabbling and reheating coils.

Since precise metering of the alloying constituent supplied by means of the treatment according to the invention is very difiicult if more than one component is added by dissolving from its oxide by means of lithium and/or calcium, either titanium or aluminum is to be added in the present example by the method disclosed by the invention, while the other constituent is introduced directly in metallic form into the melt. The kind of lining of the treatment vessel in the present example there fore depends on whether titanium or aluminum is to be present in the lining.

First, the method will be described for adding aluminum via the lining. Furthermore, lithium and/ or calcium is introduced by a method in which the material is added by means of porous carrier bodies of aluminum oxide or metal, containing lithium and/or calcium.

Producing the treatment vessel Metallurgical alumina, that is, relatively pure aluminum oxide, obtained in granular form with grain sizes of up to a few millimeters, is rammed into a metal crucible, for example, of steel. A few percent of aluminum silicon ester can be added in the form of a bonding agent to the ramming compound. Alternatively, the granular metallurgical alumina can be rammed in the dry state and a tungsten, molybdenum or graphite body can be initially inserted into the hollow space for the melt.

The lining is then heated under a protective atmosphere and by means of an induction coil to a temperature near the melting point of the metallurgical alumina, that is, to approximately 1800 C. The internal surface of the ramming compound is thus sintered so that a film of fritted aluminum oxide is produced on the surface of the treatment vessel. A fritted surface of aluminum oxide film can also be obtained by slowly and carefully melting a preliminary alloy, for example, of steel, in a steel mold which is inserted into the lined treatment vessel. As soon as the preliminary alloy is melted, the template mold also melts; the hot melt thus sinters the aluminum oxide surface.

For larger treatment vessels, the steel vessel can be lined with bricks of aluminum oxide with a calcium aluminum cement being used as a bonding agent between the bricks.

Further, a mixture of metallurgical alumina and crystalline lime can be used as a ramming compound, the proportion of aluminum oxide being determined solely by the amount of aluminum which has to be introduced into the alloy. The percentage proportion of aluminum oxide is therefore so selected as to provide an adequate amount of aluminum for the subsequent lithium treatment.

Production and charging of the porous, oxidic or metallic carrier bodies Aluminum can also be introduced into the alloy by means of aluminum oxide bodies charged with li h um and/or calcium, the reactive metal being charged on to the bodies at low temperatures so that no reaction between the lithium or calcium and the aluminum respectively takes place during the charging operation. To produce the aluminum oxide body, granular aluminum oxide of a certain grain size, for example, with a grain size of 0.8 to 1 mm., is formed into a hollow cylinder by known methods, for example, by sintering of blanks to which, in some circumstances, an organic expanding agent has been added.

A wateror air-cooled copper pipe, which functions as a retaining member, is then inserted into the inner opening of the hollow cylinder of porous aluminum oxide thus produced and is permanently joined to the sintered body. The carrier body prepared in this matter, is then immersed into a lithium and/or calcium melt under vacuum, the melt being previously prepared in a metal crucible, also under vacuum. The temperature of the lithium or calcium melt is selected so that the lithium and calcium occur in liquid form without, however, reacting with the aluminum oxide. The melt temperature for lithium is approximately 200 C.

After immersion of the carrier body, the vacuum furnace is pressurized at approximately 1.5 to 1 atm. by means of argon or helium, while the liquid lithium and/ or calcium penetrate into the pores of the carrier body and adhere therein. The carrier body is then withdrawn from the melt by means of the retaining rod to enable excess liquid to drip off and to allow the lithium and calcium contained in the carrier body to solidify.

Until required for use, the completed charged carrier body is stored in vessels in which it is protected against the ingress of air and moisture. In order to determine the amount of material charged into the carrier body, the body is weighed before and after immersion into the lithium or calcium melt.

Alternatively, the carrier body can be a porous metal body of high specific gravity, for example, of nickel, molybdenum, cobalt or tungsten, which is selected, as already mentioned, depending on whether its parent substance is permissible or even desirable for alloying purposes relative to the melt concerned. Furthermore, the specific gravity of such a carrier body should exceed that of the melt so that the body can descend without separate aids into the melt. In the present case, a carrier body of molybdenum is employed. In order to produce such a body, molybdenum powder, freshly reduced in a reducing atmosphere, for example, hydrogen, is compressed and sintered to shape. The carrier body is then externally covered with lithium under a protective atmosphere and heated to a temperature above the lithium melting point, that is, to approximately 200 C. The liquid lithium thus diffuses into the body and after solidification adheres in distributed form on the surface of the carrier body. The completely prepared carrier body is stored in the manner described heretofore.

Another method of production for a metallic carrier body, for example, of nickel, consists in the reduction to nickel sponge of commercially obtainable nickel oxide sintering bodies in directly heated mufiie furnaces, at approximately 1000 C. in a hydrogen atmosphere. These bodies are then impregnated with lithium and stored in the manner described in the first example.

Also, the carrier metal can be ground, together with lithium in a protective fluid such as carbon tetrachloride and subsequently compressed into a carrier body.

Treatment of the melt A protective atmosphere is first applied over the melt contained in the treatment vessel. In order to introduce the lithium and/or calcium into the melt, one or more of the oxidic carrier bodies prepared and impregnated in the manner described heretofore are immersed into the, melt by means of the retaining rods. The lithium and the calcium in the pores of the carrier bodies then react with the aluminum oxide of the carrier body and the vessel lining, owing to the high temperature, so that an appropriate amount of aluminum goes into solutlon.

By constructing the carrier body 1n the form of a porous body on which the lithium and the calclum adheres not only to its external surface but also to the internal free surfaces of the pores, the lithium and the calclum can be reliably introduced into the melt, although the vapor pressure of these metals at the temperature of the melt far exceeds the pressure of approximately 1 atmosphere bearing upon the melt. This effect is due to the fact that evaporation of the reactive metal from the interior of the carrier body is greatly obstructed owlng to the small size of the pores and therefore takes place with a delay in time.

In order to improve the penetration of the lithlum and/ or calcium into the melt and to achieve thorough mixing of the melt, the melt is rabbled during treatment by means of a multiphase, low-frequency alternating current of, for example, 30 c./s. and is at the same time mamtained at the correct temperature by means of med1um frequency induction coils. Other suitable means, for example, immersing electrodes or a plasma arc, may also be employed for keeping the melt at the correct temperature.

The amount of lithium to be added to the melt is deter mined primarily by the reaction equation between aluminum oxide and lithium The quantity of lithium and/or calcium added will of course also depend on the desired proportion, specified in percent by weight, of aluminum in the alloy melt.

Part of the lithium and calcium introduced into the melt will, however, react with the slag-forming agent of the melt, particularly with oxygen, nitrogen and sulphur to form lithium oxide, lithium sulphide and calcium nitride. A certain proportion is therefore added as an allowance to the stoichiometrically calculated quantity of lithium and/or calcium, in order to introduce the additional quantities required in the melt for reaction with the impurities thereof. The magnitude of the additional quantities required is based on experience and can be empirically determined. In the present case, for example, the total quantity of lithium and/or calcium introduced into the melt amounts to approximately 3% of the alloy weight.

The quantities of lithium and/or calcium can be so determined that only part of the required amount of aluminum is alloyed by reaction with the lining and the oxidic carrier bodies, while the remainder is supplied directly in metallic form.

The supplementary addition of titanium required in the present example, takes place during the lithium or calcium treatment respectively. To this end, the titanium, which is also introduced into the melt by immersion, is approximately added in the middle of a number of lithium or calcium treatments. That is, if, for example, 6 carrier bodies must be introduced to add the necessary quantities, the titanium will be added after immersion of the third charged carrier body. Titanium sponge slugs which are commercially available, are particularly suitable for alloying the titanium. The titanium slug can also function as a carrier body for the lithium to be introduced into the alloy.

Of course, the kinds of alloying for titanium and aluminum can be reversed, that is, the oxidic carrier body can be produced from titanium oxide and the treatment vessel may be lined with titanium oxide or a mixture of titanium oxide and crystalline lime in the manner heretofore described, While aluminum is introduced into the melt in metallic form.

In addition to the aluminum silicate ester already memtioned, butyl tetratitanate can be employed as a bonding agent for the lining. Aluminum is added once again in the middle of the number of lithium treatments. The stoichiometric amount of lithium in this case is determined by the equation:

Taking into account the allowance, the total quantity of titanium amounts to 0.4 to 0.5 of the alloy weight. In this case, only part of the titanium need be introduced via the lining, or the carrier bodies, while the remaining part is directly added in metallic form.

The relatively heavy charged metallic carrier bodies with which the exchange reaction between lithium or calcium and aluminum or titanium oxide takes place solely via the crucible lining, are simply thrown into the melt in order to introduce lithium or calcium therein. The carrier bodies descend, owing to their weight, without requiring any separate aids therefore.

EXAMPLE II Approximately 0.5% of titanium is to be added to the austenitic steel l88-2. To this end, the treatment vessel is lined with titanium oxide (rutile) or with a crystalline lime-rutile mixture in the manner heretofore described. 1n this case, butyl tetratitanate can be employed as a bonding agent. Also, in this case, the lithium is added by means of an autoclave in metallic form by immersion or blowing into the melt.

Treatment of the melt in the autoclave The steel is first melted in a medium-frequency induction furnace, in a crucible having, for example, a MgAl spinel lining, or in some other manner in air and teemed into the treatment vessel. After applying a reheating and rabbling coil, the treatment vessel is introduced into an autoclave. The autoclave is then evacuated and subsequently loaded with a protective atmosphere comprising argon, helium or a mixture of both, at a pressure exceeding the vapor pressure of lithium. Since the melt has a temperature of approximately 1650 C., the autoclave is pressurized at more than 6 atm Lithium may be added in the autoclave in two different Ways. In one Way, a certain quantity of pulverized or vaporized lithium is blown into the melt by means of a blowing apparatus. In another way, commercially obtainable copper cartridges filled with a known quantity of lithium powder are immersed into the melt.

The second manner of introducing the lithium offers the advantage, relative to blowing, that such requires no separate apparatus.

The quantity of lithium required is calculated primarily in accordance with the formula stated heretofore. With due consideration for the allowance, it is necessary in this case to add an amount of lithium equal to 0.3% of the weight of steel. Once again, part of the quantity of titanium can also be added directly in metallic form. Of course, the lithium can be wholly or partially replaced by calcium.

In normal precision casting, the grain size of the steel treated in this manner can be reduced in accordance with the ASTM scale from approximately 8 to approximately 4.

In both of the above examples the lithium treatment is followed by teeming of the melt into molds conventionally employed for precision casting in accordance with known methods, teeming being performed where appropriate under a protective atmosphere.

As already briefly mentioned, the purity of the alloy or steel melt is further improved by the lithium treatment. Moreover, this treatment also improves the mold filling capacity in teeming, particularly in castings where the spout usually used in teeming is endangered. These additional effects are due to the fact that lithium produces very pure metal melts. Furthermore, as lithium oxide is the only metal oxide which is liquid at the prevailing temperatures, segregation from the alloy or steel melt is facilitated. Finally, teeming of metal melts into a mold is accompanied as in known by the immediate production on the surface of a thin film of metal oxide which is usually solid. Accordingly, when metal flows into a mold, teeming is no longer accompanied by a liquid onsolid sliding but by solid-on-solid sliding. The coefficient of friction will then be substantialy higher than in liquidon-solid sliding. However, on teeming the treated alloy or steel melt of the invention into a mold, any lithium meal still present will first be oxidized so that a liquid lithium oxide film coats the surface. Owing to this circumstance, teeming of lithium-treated steel into a mold is accompanied by liquid-on-solid sliding, thus improving the mold filling capacity The method according to the invention for the adding of highly reactive elements can also be employed in the manner heretofore described for adding other elements such as zirconium, niobium, boron and tantalum.

In order to satisfy the requirements made on the purity and quality of the aforementioned steels and alloys, it was hitherto necessary to employ vacuum metallurgy techniques. However, since the application of vacuum is accompanied by considerable difliculties due to sealing of the vacuum vessel and protection of the vacuum plant against solid impurities condensed from the vapor phase, it follows that operation in vacuum is awkward and time consuming, while requiring a high technical and economic effort.

The present invention, which permits an effective lithium treatment, indicates means for avoiding the difficulties resulting from having to operate in vacuum but without, at the same time, having to impair the quality of the melt. Lithium treatment is therefore suitable for at least substantially replacing the vacuum metallurgy technique in the production and processing of high grade steels and alloys.

What is claimed is:

1. A method for alloying highly reactive alloying metals selected from the group consisting of titanium, aluminum, zirconium, boron, uranium, niobium and tantalum to metals of an alloy or steel comprising the steps of placing the melt in a vessel having a lining in contact with the melt including at least one alloying metal oxide of the highly reactive alloying metal to be alloyed in the melt; and

adding a reactive metal selected from the group consisting of lithium and calcium to the melt in the vessel, said reactive metal having a reactive property capable of reducing the alloying metal from said oxide in said lining whereby said reactive metal reacts with said alloying metal oxide to form a reactive metal oxide and to liberate the alloying metal from said lining into the melt for alloying therein.

2. A method as set forth in claim 1 wherein said reactive metal is blown into the melt in vaporized form.

3. A method as set forth in claim 1 wherein said reactive metal is immersed into the metal within a porous carrier body containing said reactive metal.

4. A method as set forth in claim 3 wherein said body is formed of an oxide of the alloying metal to be alloyed in the melt.

5. A method as set forth in claim 3 wherein said body is of a higher specific gravity than the melt.

6. A method as set forth in claim 3 wherein said reactive metal is added to the melt while maintaining the melt in a protective atmosphere.

7. A method as set forth in claim 1 wherein said reactive metal is metered into the melt to reduce a predetermined quantity of alloying metal from said alloying metal oxide into the melt.

8. A method as set forth in claim 1 wherein said lining is produced from a parent lining of a material which is chemically more stable than lithium oxide at the temperature of the melt.

9. A method as set forth in claim 1 wherein said reactive metal is added to the melt within a protective atmosphere maintained at a pressure exceeding the vapor pressure of said reactive metal at the temperature of the melt.

10. A method as set forth in claim 9 wherein said reactive metal is selected from the group consisting of lithium and calcium, and wherein said reactive metal is immersed into the melt within a carrier body containing the reactive metal in metallic form.

11. A method as set forth in claim 9 wherein said rcactive metal is selected from the group consisting of lithium and calcium and is blown into the melt within a stream of inert carrier gas.

12. A method as set forth in claim 1 which further comprises the step of stirring the melt during said step of adding the reactive metal under the influence of a lowfrequency multiphase alternating current.

13. A method as set forth in claim 1 which further comprises the step of adding another highly reactive alloying metal into the melt simultaneously with said step of adding said reactive metal.

14. A method as set forth in claim 1 which further comprises the step of adding another highly reactive alloying metal into the melt subsequent to said step of adding said reactive metal.

References Cited HYLAND BIZOT, Primary Examiner T. R. FRYE, Assistant Examiner US. Cl. X.R. 

